Conventionally, image forming apparatuses each forming an image on a sheet-like recording medium such as a paper sheet by using recording agents including a decolorable coloring material have been developed. For example, such an image forming apparatus is a multifunctional device such as a multifunctional peripheral (MFP). A recording agent (hereinafter, referred to as a decolorable recording agent) including a decolorable coloring material can be decolored by being heated to a predetermined temperature (hereinafter, referred to as a decoloring temperature). In addition, decoloring devices each decoloring an image formed on a recording medium by the image forming apparatus described above have been developed. A conventional decoloring device executes a decoloring process constantly at a decoloring temperature of a same value, and accordingly, there are cases where the power consumption increases.